1. Field of Invention
The present invention relates to an active matrix liquid crystal light valve. More specifically, the present invention relates to a high aperture ratio active matrix liquid crystal light valve which operates in a reflective mode, which modulates the polarization of incident light, and which produces very bright, high contrast ratio projected images.
2. Summary of Prior Art
Spatial light modulators consisting of a liquid crystal layer which is addressed by an active matrix of electronic elements can be used to modulate light. Active matrix liquid crystal light valve cells (hereafter simply referred to as either as cells or light valves, without mention of the addressing matrix) are undergoing extensive commercial development primarily driven by the desired use as direct view displays for commercial television applications and personal computing products. Direct view displays are ones in which the cell itself is viewed, as opposed to projection displays, in which the image projected from the cell is viewed. The direct view displays generally use transmissive rather than reflective light valves. Transmissive light valves allow backlighting for improved contrast, color quality, and visibility in low ambient light. Less development effort has been applied to projection cells, and the work that has been done tends to leverage off the intense development of direct view displays. As a result, prior art light valves developed for projection displays have certain deficiencies.
Two of the most important properties of projection displays are luminance and contrast ratio. High luminance is required in order to provide images which are not washed out by ambient light. High contrast ratio is required for good greyscale and color tones (a contrast ratio of 100:1 is needed for faithful reproduction of the full range of observable greyscale tones). Aperture ratio and contrast ratio are more critical for projection devices than for direct view devices. Aperture ratio is the ratio of the light modulating area of the light valve to the total area of the light valve. For a given projection system, luminance will be directly proportional to the aperture ratio of the light valve. Because large optical elements are prohibitively expensive, the size of projection cells must generally be less than that of direct view cells in order to enable the use of small and low cost projection optics. Therefore, to achieve the same information content, a higher density of pixels is required on the cell. The higher pixel density results in a lower aperture ratio, because the area required for transistors, pixel isolation, and row and column leads is relatively constant. Typical aperture ratios of transmissive projection light valves are less than 50% for 240 line devices and less than 20% for 1000 line devices. Attempts to improve aperture ratio by making narrow leads and reducing pixel spacing typically reduce manufacturing yield and increase cost. Other approaches, such as utilizing optical elements as disclosed in U.S. patent application Ser. No. 480,270 by Zampolin et al., and assigned to the same assignee as the present invention, increase system complexity and therefore have a cost impact as well.
Prior art projection systems also generally utilize transmissive light valves because this enables them to be made on the same manufacturing lines as the direct view devices. Another benefit of transmissive projection light valves is simplified and hence lower cost projection optics. U.S. Pat. No. 4,764,390 by McKechnie et al., is an example of one projection system that utilizes transmissive light valves. However, there are numerous drawbacks in the use of transmissive light valves which can be avoided by using reflective light valves. Obviously, transmissive light valves must utilize transparent materials in the addressing matrix of electrodes. Glass substrates can provide a large area substrate at reasonable cost. In prior art the active matrix is composed of amorphous silicon (a-Si) or polycrystalline silicon (poly-Si). Amorphous silicon uses low temperature processing and thus can use a low cost glass substrate. Polycrystalline silicon uses a higher temperature processing, but the improved electrical properties of poly-Si allow integration of the matrix address and drive electronics on the same substrate.
A prior art reflective light valve fabricated using a-Si active matrix liquid crystal display technology on a glass substrate is revealed in "High Density Reflective Type TFT Array for High Definition Liquid Crystal Projection TV System" by Takubo et al., in JAPAN DISPLAY 1989 which uses a-Si Transistor Switches. The reflective electrodes of this array must be polished mechanically to obtain a mirror surface. This is an intrinsically time consuming and low yield process. Increased aperture ratio can be obtained because the TFTs and well as the row and column leads can be located under the reflector electrode and therefore do not have to increase the non modulating area. Even so, the quoted aperture ratio is limited to 70%.
Examination of prior art indicates that there are many problems with the use of a-Si and poly-Si circuits. The field effect mobility of a-Si (and hydrogenated a-Si) transistors is usually too low to integrate the drivers into a functional video device. Photoconduction, 200 times higher in a-Si than poly-Si, is a large problem in devices which use a-Si because they require special light shields or blockers. In poly-Si circuit devices, ordinary substrates cannot withstand the high temperature processing, and therefore fused quartz or a high-melting point glass is used instead of ordinary soda lime glass as substrate. These temperature resistant substrates are very expensive. Also, differences in thermal expansion coefficients can cause cracks in the recrystallized poly-Si layers. However, the most important problem with the prior art is that the fabrication of a-Si and poly-Si circuits on glass are not conventional integrated circuit processes and therefore require specialized manufacturing lines. This results in lower yields than could be achieved if the more mature process technology and equipment of conventional integrated circuit production lines were to be used. Furthermore, major investments, typically $30 million to $300 million or more, are required to develop and build specialized a-Si and poly-Si circuit production lines.
Production lines for direct view devices have been specialized, and modification of these lines for the production of the higher resolution and smaller area projection cells would be costly. Construction of specialized lines to produce projection cells would also be very expensive. A prior art alternative is active matrix light valves based on the use of silicon metal-on-oxide field effect transistors (MOS transistors) fabricated on a silicon monolithic wafer. MOS is mainstream integrated circuit technology, and the MOS manufacturing technology is mature. Because the silicon wafer is opaque, such devices are limited to reflective mode operation.
The prior art of reflective light valves using MOS technology is described in the book "Liquid Crystal TV Displays: Principles and Applications of Liquid Crystal Displays", by E. Kaneko (1986). The important problems discussed by Kaneko in this prior art are low aperture ratio, rough reflection electrode surfaces, nonglanar topographies, and interface reflections. As previously documented, low aperture ratio limits luminance. Rough surfaces can cause increased light scattering which limits contrast ratio. Non-planar topographies can limit the thickness tolerance of the liquid crystal layer and also produce contrast ratio reducing light scattering and spurious reflection. Very uniform liquid crystal layer tolerance is required for both high contrast ratio and high luminance. Finally, reflections from the counter electrode and its substrate also reduce contrast ratio.
Additional prior art, represented by U.S. Pat. No. 4,239,346 by Lloyd, presents a reflective light valve which utilizes MOS transistors and which operates in a scattering mode. In this device, the capacitor and drive lines are also placed beneath the reflective electrode. This prior art solves part of the problems of nonglanar topography by placing most of the reflective electrode above an insulating layer that also serves as a leveling layer and also provides higher aperture ratio than other cells in which the busses are coplanar with the electrodes. However, the reflective/conductive electrode still has an uneven surface and gaps between the electrodes which limit aperture ratio. Finally, the device has interface reflections which tend to reduce contrast ratio.
An additional problem in most prior art addressing matrices is the necessity of storage capacitors. The liquid crystal materials used for scattering mode operation must have relatively low resistivity. Consequently they have a low RC constant, and quickly discharge after removal of an applied field. The storage capacitors serve to increase the dielectric relaxation time until it is longer than a frame time, which is about 33 ms for a 2:1 interlaced 60 field/second video signal. The capacitors retain charge so that a steady field is applied across the liquid crystal for a duration up to a frame time, resulting in improved luminance and eliminating flicker. These capacitor layers add complexity and cost to the construction of the light valve.